The present invention has been developed to be employed in the moulding of thermoplastic package parts directly on other package parts, such as for example the moulding of a thermoplastic package top on a sleeve of packaging laminate including a core layer of paper. The package top may, in addition to being moulded against the sleeve, also be moulded against an opening device in the form of a cap, for example a screw cap.
The result will be a packaging container in which food may be packed. The Applicant markets similar types of packages under, for example, the registered trademark Tetra Top®.
Such a packaging container is manufactured today in that a flat-laid, tube shaped blank of a packaging laminate is raised and sealed in its one end in that a top of thermoplastic is injection moulded direct on the end portion. The top includes a pouring opening which is defined by a neck with outer threads for engagement with inner threads of a screw cap for sealing the pouring opening. The cap may be applied in two ways. As was mentioned by way of introduction, it may be placed in the moulding tools so that the top, in addition to being moulded against the sleeve, is also moulded against the cap. In such instance, the cap is part of the moulding tools. Alternatively, the cap may be applied once the packaging container is ready formed, i.e. no moulding with the cap as part of the moulding tools takes place. Possibly, in order to facilitate handling in the packing and filling machine, the pouring opening of the neck may, in the latter case, be formed with a tear-off membrane which is moulded simultaneously with the top.
The moulding technology which is currently employed in this context is that which is known as injection compression. In injection compression, a partly closed injection moulding cavity is partly filled with molten thermoplastic material, in a first step. The cavity is then closed completely which gives a compression force on the melt inside, the melt then coming to fill out the cavity. This technology is described in the Applicant's own patent publication U.S. Pat. No. 5,667,745.
The package parts which are injection moulded are of thin-walled material and the production cycle time for this type of injection moulding which takes place in a packing and filling machine is extremely short compared with normal injection moulding. Typically, the production cycle time for the manufacture of one top should not be longer than 1 second. This is necessary in order to be able to maintain machine output capacity at a high level.
From the injection moulding, the package is conveyed with its open end directed upwards and is filled in a subsequent filling station. In some cases, the package also passes one or more sterilization stations before filling. After filling, the open end of the package is folded and sealed.
One alternative to injection compression is compression moulding. In this operation, an adapted quantity of molten thermoplastic material is injected into a cavity in open moulding tools, whereafter the moulding tools are compressed together so that the melt will fill out the cavity.
In order to obtain a good result in both of these types of moulding of thin-walled package parts, it is of crucial importance that the plastic melt is rapidly distributed uniformly in the cavity when the pressure is applied. This becomes particularly important in those cases where the configuration of the package parts is complicated and in those cases where the intention is to manufacture more than one package part at the same time in the same moulding tool.
In conventional injection moulding and compression moulding, use is made of stable suspension of the moulding tools and symmetry is required in the moulding apparatus to obtain a good quality of those objects which are moulded. For example, it is necessary that the moulding tools be placed in a symmetric pattern (in, those cases where the intention is to have more than one cavity) so that the injection ducts on injection moulding will be of equal length, that the same quantity of melt is injected in those points at which the melt is fed into the cavity or cavities and that the points are uniformly distributed. However, despite the symmetry it may be difficult to attain balance in the moulding apparatus, i.e. to cause the flow front of the plastic melt to move uniformly everywhere before the plastic melt has time to harden anywhere. External disruptions may easily give rise to instability. Such disruptions may, for example, be uneven control of the moulding tools or changes in the quantity of plastic melt at any point or in the composition of the plastic melt proper.
One way of realising greater stability and greater immunity to disruption is to use moulding tools which have a certain mutual movability in relation to one another also once the compression has commenced. This means that at least the one moulding tool is suspended more or less “buoyant” and can compensate for certain disruptions so that stability and power balance within the moulding apparatus can nevertheless be attained.
It has however proved that this solution per se is insufficient to achieve optimum moulding, but there is room for improvement as regards balancing of the mould.